The present invention is directed to fiber optic transport systems. Embodiments of the invention provide methods and system for improving reliability of fiber optic transport system using an interleaving dual thermoelectric coolers (TEC) configuration. Merely by way of example, the invention has been applied to dense-wavelength-division-multiplexing (DWDM) fiber optic transport system with integrated transmitter/receiver module. But it would be recognized that the invention has a much broader range of applicability.
Since its first deployment in the middle of 1990s, dense wavelength division multiplexing (DWDM) has become a dominant technology for long haul and regional backbone transport networks, and is gradually making its way to metro area networks. In a conventional DWDM system, each optical component, be it a laser or a MUX filter, is individually packaged. A linecard is built around one or several optical components. For example, a transmitter card for a given wavelength includes a laser and a modulator (or an integrated laser/modulator). The laser chips sitting inside the laser packages are typically made of indium phosphide (InP) semiconductor compounds. The optical outputs of multiple transmitter linecards at different wavelengths are combined through a multiplexer linecard, which includes some MUX filters. A commonly used MUX filter is based on array waveguide grating (AWG) made of silica-on-silicon. The optical connections between the linecards are through optical fibers. The optical output from the multiplexer linecard is then amplified by an erbium doped fiber amplifier (EDFA) and launched into the transmission fiber. The optical outputs from the demultiplexer linecard, each at a wavelength by the ITU-T standards, are then fed into the receiver linecards. The optical connections between the linecards are generally made through optical fibers. A receiver card typically includes a photodetector, for example, a p-i-n (PIN) photodiode, or an avalanche photodiode (APD), that converts the input optical into an electrical signal for further processing. The photodetector chips inside the photodetector packages are typically made of InP semiconductor compounds.
Extensive efforts have been devoted recently to the integration of multiple transmitters/receivers along with AWG-based multiplexers and demultiplexers for its use in DWDM transport applications as a single integrated Tx/Rx module with the intent to reduce system footprints as well as the cost of Optical-Electrical-Optical (OEO) converter system. For example, ten transmitters and/or receivers, each operating at 10 G, have been successfully integrated in to a single line card with only one pair of fibers for line side connection. One of the drawbacks is its low yield and hence high cost. Different integration schemes have been proposed, including hybrid integration on silica/silicon which shows the promise of reducing the manufacturing cost.
One of the important applications of the integrated Tx/Rx module is to replace existing EDFAs in an optic network to have an OEO converter at every point of presence (PoP). This replacement has numerous advantages including the elimination of optical power management in the physical layer, simple and efficient network protection, and its flexibility/scalability. However, without protection, full OEO converter including such integrated Tx/Rx module will have much lower reliability than that of an EDFA. Even if 1:N protection is provided, there is still a need to further improve the reliability of the integrated Tx/Rx modules for them be used to replace EDFA at every PoP. The reliability of such integrated Tx/Rx module needs to be at least comparable to or better than that of EDFA to make the above replacement viable.
As a matter of fact, the total downtime of the DWDM fiber optic transport system has been bottlenecked due to the reliability of the integrated Tx/Rx modules which often need temperature control using thermoelectric coolers (TEC) because the lasers and the photo receivers work much better at lower temperatures. The failure mechanism for these devices is often associated with the failure of the TECs which eventually leads to malfunction or damage of the laser chips and receiver diodes.
FIG. 1 is a schematic view diagram of a conventional TEC module. As shown, the TEC module includes two plates made of heat conductive ceramics that is also a good electrical insulator. As shown, a series of P- and N-doped thermoelectric semiconductor elements or blocks is alternatively disposed and sandwiched between the two plates 101 and 103. Most TEC module is fabricated with a equal number of N-type and P-type blocks where one P and N block pair form a thermoelectric unit or couple which is usually called a Peltier diode. The series of Peltier diodes or alternating P-type and N-type thermoelectric semiconductor blocks are connected electrically in series and thermally in parallel. For example, any P-type/N-type block in the series may be connected its one end electrically, by a conductor 111 that is in thermal contact with one plate, to a corresponding end of only one neighboring N-type/P-type block. The same P-type/N-type block may be connected its opposite end electrically, by another conductor 113 that is in thermal contact with the other plate, to a corresponding end of another neighboring N-type/P-type block. When a DC voltage is applied to the TEC module, it causes a DC current to flow from an in-terminal 121 to an out-terminal 123, through each P-type block in a direction from one plate to another and alternatively through each N-type block in an opposite direction, resulting a Peltier effect, i.e., a current induced net heat transfer from one plate to another. As shown in FIG. 1, the TEC module in operation would make one plate 103 hot and the other plate 101 cold. Reversing the voltage polarity will reverse the heat transfer direction the first plate will now get cold and the second plate turns hot. When applying the TEC for active cooling, the “hot” plate 103 of the TEC module usually is mounted onto a heat sink, while the cold plate 101 of the TEC module is used for mounting a target system whose temperature needs to be controlled.
However, the TECs often have relative high failure rate. As a result, the failure rate of the integrated Tx/Rx module may be limited by the TECs used. Therefore, in order to make the integrated Tx/Rx module viable versus EDFAs in the optical transport system, it is desirable to improve the reliability of the thermoelectric cooler so that the full OEO converter reliability is at least comparable to or better that of an EDFA unit. Particularly, redundancy is considered to be useful to enhance the reliability.
As seen above, TEC failure is a bottleneck in reliability of the integrated Tx/Rx module in optical networks with frequent OEO conversions. Therefore, improved techniques for increasing TEC reliability are desired.